The objective of this proposal is to elucidate the molecular and biophysical mechanisms that regulate platelet adhesion to disrupted vessel wall at sites of injury. The focus is the interaction of the platelet glycoprotein Ib (GPIb) with plasma protein von Willebrand factor (VWF) that is bound to collagen on subendothelial extracellular matrix. The GPIb-VWF interaction mediates the initial step of the multistep adhesion and signaling cascade, which includes platelet tethering to and rolling (or translocating) on disrupted vascular surfaces. Regulation of this initial adhesion process is crucial: Insufficient adhesion cannot stop bleeding to maintain hemostasis. Excessive adhesion results in thrombosis. Our hypothesis is that initial platelet adhesion is regulated by the interplay of biophysical parameters of the blood flow with specific kinetic and mechanical properties of the GPIb-VWF interaction. Mutations in GPIb and VWF, such as those naturally occurring in patients with various types of von Willebrand diseases (VWD), alter these kinetic and mechanical properties as well as their regulation by biophysical parameters, thereby changing platelet adhesion and resulting in thrombotic or bleeding disorders. Using combined experimental, computational, and theoretical approaches, this hypothesis will be tested in three integrated specific aims: 1) Quantify the regulation of GPIb-VWF interaction by biophysical parameters, 2) Determine the regulation of GPIb-VWF interaction by structural variations, and 3) Investigate GPIb-VWF interaction by molecular dynamics simulations. This integrated and systematic study will clarify how the kinetic and mechanical properties of GPIb-VWF interaction fulfill the biophysical requirements for platelets to adhere to blood vessel wall in the mechanically stressful environment of the circulation. Decoding how molecular structure determines these properties and their regulation by biophysical parameters will provide key insights into vascular physiology and pathology. As a result, the data may offer new therapeutic approaches to inhibiting pathological platelet adhesion during thrombosis and/or intervention to the bleeding disorder. We propose to elucidate the biophysical mechanisms that regulate the molecular interaction of the platelet glycoprotein Ib (GPIb) with protein von Willebrand factor, which mediates platelet adhesion to disrupted vessel wall at sites of injury. This regulation is crucial because insufficient adhesion cannot stop bleeding to maintain hemostasis but excessive adhesion results in thrombosis. The data may offer new therapeutic approaches to inhibiting pathological platelet adhesion during thrombosis and/or intervention to the bleeding disorder von Willebrand diseases.